Idas? olympicus, Kiel & Goedert, 2007

Kiel, Steffen & Goedert, James L., 2007, New mollusks associated with biogenic substrates in Cenozoic deep-water sediments of Washington State, Acta Palaeontologica Polonica 52 (1), pp. 41-52 : 47-48

Idas? olympicus sp. nov.

Fig. 6 .

2006 Idas n. sp.; Kiel and Goedert 2006a: 2626, fig. 2b.

2006 Idas n. sp.; Kiel and Goedert2006b: 549, fig. 2B.

Derivation of the name: For the Olympic Peninsula in western Washington State.

Holotype: USNM 532038 . Well−preserved large specimen, inner side concealed by matrix.

Paratypes: USNM 532039–532041 from USGS loc. 26905, USNM 532042 from USGS loc. 26898.

Type locality: USGS loc. 26898, Murdock Creek , Clallam County, Washington State, USA .

Type horizon: Upper lowermost Oligocene, lower part of the Pysht Formation.

Material.— The type material and numerous specimens associated with wood−fall sites USGS locs. 26895, 26896−A, D, 26897−A to G, 26898−A to J, 26899, 26900, 26901, 26902, 26904; and with the whale specimens USNM 314593 and 314608 from USGS loc. 26896, and USNM 314545 , 314556 at USGS loc. 26898. All specimens are deposited in the Cenozoic mollusk collection of the USNM.

Diagnosis.—Prodissoconch with round ventral margin and prominent, triangular umbo; adult shell modioliform, beak strongly prosogyrate, in subterminal position; posterodorsal margin straight or slightly convex, transition to posterior margin at an angulation at between 50 and 62 percent of entire shell length; ventral margin straight or slightly concave; posterior hinge area moderately broad, showing small denticles with 45 ° inclination; shell surface shows fine growth lines.

Description.—Prodissoconch 560 ̊m long and high, whitish, posterior side marginally more pointed than anterior side, margin flattened. Juvenile shell (1.3 mm) elongate−triangular in ventral view, highest at about three−quarters of total length, hinge line straight, posterior margin slightly angular at its middle, ventral margin straight to slightly convex, anterior margin short, pointed. Shell composed of three layers: a calcitic outer, homogenous or foliated layer with the sheets perpendicular to the shell’s surface; a nacreous middle layer, and an inner layer of simple prismatic structure. The holotype is 17 mm long and 7 mm high, the largest specimen is 20 mm long.

Discussion.—The mineralogic composition of the shell (calcite and aragonite) was confirmed by X−ray analysis. Distinguishing Idas from small Modiolus on the limited shell features available for Idas? olympicus is difficult, because they can have very similarly shaped larval and adult shells, and also their hinge dentition can be similar. Ongoing work by SK and Luciana Génio (unpublished) indicates that Modiolus usually has a proportionally thinner outer calcitic layer than Idas . However, due to the poor preservation of the material available, our specimens are assigned to Idas with some hesitation.

Idas pelagicus (Woodward, 1854) , I. osseocolus ( Dell, 1987) , I. pacificus ( Dall, Bartsch, and Rehder, 1938) , I. coppingeri ( Smith, 1885) , and I. washingtonius ( Bernard, 1978) all differ from I.? olympicus by having the umbo in a more central position. Idas japonicus ( Habe, 1976) , I. ghisottii Warén and Carrozza, 1990 , and I. argenteus Jeffreys, 1876 have the umbo in an anterior position similar to I.? olympicus , but differ by having a shorter posterior margin relative to the ventral margin. Idasola ? sp. from an Oligocene whale−fall in Washington State has the umbo in a more central position, and has a more curved outline in ventral view (Goedert et al. 1995). The same is true for Adipicola chikubetsuensis ( Amano, 1984) from a Miocene whale−fall in Japan ( Amano and Little 2005). The “modiolids” reported earlier from whale−falls in this area (Goedert et al. 1995) most likely also belong to Idas? olympicus , as well as the “ Adula ” reported by Lindberg and Hedegaard (1996) from wood−fall concretions containing the acmaeoid Pectinodonta palaeoxylodia Lindberg and Hedegaard, 1996 . Idas? olympicus may represent the earliest record for the genus.

Stratigraphic and geographic range.—Uppermost Eocene to lowermost Miocene, Makah, Lincoln Creek, and Pysht formations, Washington State, USA. Associated with wood and whale bones.

Figures

Fig. 6. Mytilid bivalve Idas? olympicus sp. nov. from Eocene to Miocene whale− and wood−falls in Washington State. A. Paratype USNM 532039, juvenile shell showing larval shell (A1), from a uppermost Eocene wood−fall at the Satsop River (USGS loc. 26905), and two close−ups on the prodissoconch (A2, A3). B. Holotype USNM 532038, from a upper lower Oligocene wood−fall at Murdock Creek (USGS loc.26898−A).C. Paratype USNM 532042, from an upper lower Oligocene whale−fall at Murdock Creek (USGS loc. 26898), showing shell microstructure (h = homogenous calcite, n = nacre). D. Paratype USNM 532040, showing hinge dentition anterior to umbo, from same locality as C. E. Paratype USNM 532041, showing hinge and hinge dentition, from same locality as C.